Electrical switches are commonly used to control the flow of current in electrical circuits.
Common types of electrical switches comprise mechanical contacts that can be made to open or close by manual operation or in response to an actuating mechanism, such as electrical actuation, magnetic induction, thermal activation, etc. These types of electrical switches, also called mechanical switches, can be found in various switching devices such as relays, circuit breakers, and ground fault interrupts.
Without further measures, normal switches could only separate 12 to 20 V DC. However, even if this limit can be shifted to higher voltages by the application of external magnets, the power dissipation in the unavoidable arc when the switch contacts are separated erodes the contact material and therefore limits the lifetime of the switching device.
The high temperatures reached during arcing may also cause melting of the contact portions or transfer of material between contacts which result in contact wear. The contacts may develop uneven surfaces that mechanically lock the contacts when the switch is operated to open.
Another undesirable effect of arcing is the contamination of the areas surrounding the switch due to the evaporation and sputtering of contact material.
The overheating associated with arcing might also damage the surrounding areas and lead to the destruction of the device.
Arcing is particularly important in switching devices such as high voltage relays used for protecting electrical circuits from faulty conditions and/or disconnecting them from high voltage power supplies.
The high electric field established across the air gap between the switch contacts when these are separated for interrupting the supply of high voltage power to an electrical load produces an intense arc current between the separated contacts that may destroy the switch as well as the circuits to be protected.
Thus, it is desirable to limit the effects of arcing as much as possible such as to improve the reliability and lifetime of the mechanical switch as well as to avoid destruction and/or device contamination.
Several measures have been proposed for protecting relay contacts and which rely on dissipating the high energy generated across the opened relay via arrangements of electric components, such as resistors, diodes, capacitors, connected in series or in parallel to the relay contacts. The suitable arrangement depends on the type of relay and its specific application.
A positive temperature coefficient of resistance device, also called a positive temperature coefficient thermistor or simply PTC device, such as the devices sold by Tyco Electronics Corporation under the trademark PolySwitch, is another example of passive component that has been proposed for protecting contacts from arcing.
PTC devices are generally used for providing electrical circuit protection against faulty conditions, such as overcurrents through the PTC device or excessive surrounding temperatures. Commonly used PTC devices are based on conductive polymer compounds.
The interesting characteristic of these devices lies in their non-linear resistance behavior. A PTC device has a current rating, above which it changes from a low temperature, low resistance state, also called the on-state or un-tripped state, into a high temperature, high resistance state, that causes the current flowing through the PTC device to be greatly reduced. The PTC device is then said to be in a tripped state or simply “tripped”.
The rated trip current may vary from 20 mA to 100 A, depending on the type of PTC device. The transition to the tripped state may also occur if a current larger than the trip current is maintained through the PTC device for more than a given time.
In order to return the PTC device to the low resistance state, the PTC device has to be disconnected from the power source and allowed to cool, even if the current and/or temperature have return to normal levels.
U.S. Pat. No. 5,864,458 describes an example of overcurrent protection system that permits the use of mechanical switches and PTC devices to switch voltages and currents in normal circuit operations, while the voltage and/or current ratings of the mechanical switches and PTC devices are much less than the normal operating voltages and currents of the circuits.
The overcurrent protection circuit comprises a PTC device connected in series with a load, and a bimetal switch connected in parallel with the PTC device, which are thermally coupled.
The PTC device and bimetal switch serve to limit the fault current delivered to the circuit. In case of an overcurrent, the bimetal switch heats and opens, shunting the current to the PTC device. The overcurrent in the PTC device causes the PTC device to quickly trip to its high resistance state, reducing the current to a very low level. The low current in the PTC device keeps the PTC device heated and in a high resistance state. The heat from the PTC device latches the bimetal switch in the open state, preventing oscillation of the contacts of the bimetal switch.
By shunting the current to the PTC device, the contacts of the bimetal switch do not arc since they do not have to switch the current at operating voltage.
U.S. Pat. No. 5,737,160 proposes electrical switch arrangements for interrupting a current and voltage higher than the rated currents and voltages of each of the switches and the PTC devices.
The electrical switch arrangements comprise two mechanical switches in series or in parallel, and a PTC device which is connected in parallel with one of the switches (referred to as “the parallel switch”), and in series with the other switch (referred to as “the series switch”).
The design of the arrangement depends on the speed at which the resistance of the PTC device increases. If both switches are operated simultaneously, the current will continue to flow through the series switch, in the form of an arc between the contacts, until the increasing resistance of the PTC device reaches a level such that the arc is not sustained.
The use of a PTC device that quickly reaches that level may lower the required rating of the series switch.
If the series switch is operated after the parallel switch, the duration of the arcing in the series switch may be reduced and/or completely eliminated. Thus, there will be no arcing in the series switch if the resistance of the PTC device reaches the required level before the series switch opens.
However, a problem remains on how to ensure that the delay between the operation of the two switches is sufficient but not longer than required for suppressing arcing.
For instance, if the series switch is not operated (i.e. opened) as soon as the resistance of the PTC device reaches the required level, the PTC device must be able to sustain the full voltage in a high temperature state, without damaging itself or any other component, until the series switch is operated. Otherwise, the PTC device may be damaged or cause damage to other components.
The series switch should open and/or be operated shortly after the parallel switch for ensuring that the circuit is not live for an appreciable time after the parallel switch has been operated.
In order to avoid this problem, the characteristics of the PTC device and the rated voltage of the switches are selected so as to control the speed at which the PTC reaches the required level. However, this has the disadvantage that the electrical switch arrangement must be customized for each specific application.
In particular, the characteristics of PTC devices may change considerably among devices of the same type. Thus, a switching mechanism that allows for compensation of changes among PTC devices would also be desirable.
Finally, although the above proposed measures allow the reduction of the effective current/voltage at which the switches are opened for avoiding arcing, at present there are no solutions regarding how to control the time delay between operations of the switches and how to synchronize the switching tripping of the PTC and the galvanic isolation sequence.